1. Field of the Invention
The present invention relates generally to a mechanical joint structure for a brace for the knee, a natural slide and hinge joint in the human body. More particularly, it relates to a joint structure which is slidably-coupled and which has means for constraining the articular motion of the pivotally interconnected joint elements to follow a definite path when they are bent within the degree of flection normally encountered during walking. The invention has specific application as a load-supportive and motion-controlling joint in the rehabilitation of a damaged or diseased knee joint and in prostheses.
2. Description of the Prior Art
Slidably-coupled joint structures having a single pivot for use in knee braces and the like have been described in the prior art. Such structures may include stops for limiting the relative angular movements of the pivotally interconnected joint elements to protect the wearer against hyperextension of the knee joint and against its flection beyond the norm. However, in order to minimize the possibility of premature locking of the joint between its extreme extended and retracted positions, the articular motion is composed of an indefinite combination or rotation and translation. This absence of positive control, while it may be desirable in a brace intended to protect the healthy knee joint of an athlete participating in contact sports, can lead to the collapse of a weakened or diseased knee joint. Such a collapse may be precipitated by the accidental shifting of a person's weight, while standing, onto a leg braced by one of these structures.
Moreover, the tendency of the knee braces of the prior art, including slidably-coupled joint structures with a single pivot, to migrate up and down a wearer's limb makes them less than satisfactory for use in orthopedic devices such as mobile casts. Such a cast must allow a patient to move his knee joint naturally so that he can exercise an affected limb in order to reduce joint stiffness and muscle atrophy; and, at the same time, the cast must prevent the compression and separation of a fracture altogether to insure its proper mending and in particular that the bone does not offset. The slidably-coupled joint structures of the prior art cannot perform both of these functions. Indeed, some of these devices, including those disclosed in U.S. Pat. Nos. 3,552,786 and 4,320,747, shorten when the pivotally interconnected joint elements therein are bent rather than lengthening with the leg as it is being flexed.
The acute need for a joint structure which lengthens a small amount as the leg first starts to bend and shortens when the leg is straightened as does the natural leg exists not only for walking casts but also among prosthetic devices. At present, a person fitted with an artificial limb which does not shorten as it is straightened must cope with a limb which is longer than his natural one. That is, he has to raise himself over the prosthesis, passing it beneath his torso, and then more or less fall forward. Repeating the process, he must position the prosthesis in front of himself once again by moving it so as to cause the joint to bend and then slinging it with enough force to straighten the joint before the heel strikes the floor. If there is too much friction between the joint elements, the requisite bending motion cannot be achieved; and the user has to swing the prosthesis around to his side, adding to his fatigue and creating an even more awkward gait.
A knee joint for an artificial leg which does shorten as the leg is straightened is disclosed in Great Britain Pat. No. 691,264. Unfortunately, this prior art device also lengthens throughout any motion increasing the amount of bending of the joint. Moreover, a person wearing it cannot squat; and in order to sit in a chair, he must begin by sitting sideways with the prosthesis extending straight out or in a slightly bent configuration, then turn so as to position the device in front of himself, and finally lift its thigh portion with his hands so that its shin portion can move under its own weight, bending the knee joint. Even when he has managed to seat himself properly, the wearer of this knee joint still has to contend with the undesirable feature of its being noticeably higher than his natural knee.